A significant amount of energy is wasted when hot products of combustion generated in industrial furnaces are vented as flue or exhaust gases. A number of techniques have been developed to recover at least part of this waste energy.
Regenerators, for example, provide a cyclic heat interchange, alternatively receiving heat from outgoing hot gaseous products of combustion and transferring it to, and thus preheating, the incoming combustion air. Typically, regenerators have a heat reclamation bed made of or filled with a packing material that stores and transfers heat. While large checkerwork refractory regenerators have been known for decades, a more recent development has been the introduction of integral burner-regenerators, also known as regenerative burners.
In general, regenerative burners are provided in pairs, with one unit operating in a combustion mode and the other in an exhaust or flue mode. For twin units A and B, for example, unit B may be operated as a burner while hot flue gases are cooled by being passed through the bed of unit A which is operated as "flue". When the bed of unit A has reached the targeted temperature, the flue gases are redirected to the bed of unit B, now operating as flue, while unit A is switched to burner mode; heat stored in the bed of unit A is recovered as the combustion air at ambient temperature is passed through the hot bed and is preheated. Once the bed of unit B reaches the targeted temperature, unit B is again switched to burner mode while hot exhaust gases are redirected to unit A.
Although it is known to recover waste energy from hot flue gases by preheating incoming combustion air, this preheating approach is not normally practiced in oxygen-based combustion processes where the oxidant is typically employed at ambient temperature. One reason is that the energy savings expected from preheating oxygen are modest. Moreover, there are a number of technical difficulties associated with handling hot combustion oxygen. Although oxygen may be preheated using indirect heat exchangers or recuperators, such units have limitations imposed by the materials employed in their construction; generally, the oxygen preheat temperature that can be reached in such heat exchangers does not exceed about 1300.degree. F.
Problems also exist with the attempt to preheat oxygen using the rapid cycle regenerators currently available for air-fired furnaces. For example, the beds of these regenerator systems become plugged when the flue gases contain dust and/or condensables; consequently their use is limited to relatively clean processes.
Since the typical cycle time of a rapid cycle regenerator is less than 2 minutes, the size of the beds in these units is small. In the case of oxygen preheating, for a flue gas temperature of about 2400.degree. F., flue gases exiting the regenerator remain at an excessively high temperature, about 1500.degree. F., as compared to only about 300.degree. F. for preheating air. Moreover, the volume of residual oxygen left in the regenerator at the end of the preheating cycle may be as high as 5% to 10% of the oxygen flow volume per preheating cycle. When the flow is reversed, this residual oxygen is lost in the exhaust. Generating increased amounts of NO.sub.x by combusting high temperature oxygen is yet another technical problem that may arise from oxygen preheating.
Clearly, the special characteristics of oxy-fuel combustion impose limitations on the possible approaches to heat recovery, limitations that are not encountered in conventional processes where preheating the combustion air is cost effective, well understood and widely practiced.
Accordingly, it an object of the invention to provide a process for improving the recovery of waste energy from hot flue gases produced in furnaces employing regenerative beds.
It is another object of the invention to provide a process for improving the recovery of waste energy from hot flue gases produced during the combustion of a fuel with an oxidant having an oxygen concentration higher than that of air.